between last chapter and this chapter, i pretty much conclude that the egg is to erase watanuki's memories, potentially the entire world's memories, of yuuko. that way watanuki could live his life as a healthy, normal boy, which is what i'm sure yuuko wanted, and that choice she gave to doumeki. it seems like he's hesitant about doing this, because watanuki's so adamant about succeeding the shop, but..i have a feeling the egg might be used. just to end the story for good.